California Penal Code § 1328d

Notwithstanding Section 1328, a subpoena may be delivered by mail or messenger. Service shall be effected when the witness acknowledges receipt of the subpoena to the sender, by telephone, by mail, over the Internet by e-mail or by completion of the sender’s online form, or in person, and identifies himself or herself by reference to his or her date of birth and his or her driver’s license number or Department of Motor Vehicles identification card number. The sender shall make a written notation of the identifying information obtained during any acknowledgment by telephone or in person. The sender shall retain a copy of any acknowledgment received over the Internet until the court date for which the subpoena was issued or until any further date as specified by the court. A subpoena issued and acknowledged pursuant to this section shall have the same force and effect as a subpoena personally served. Failure to comply with a subpoena issued and acknowledged pursuant to this section may be punished as a contempt and the subpoena may so state; provided, that a warrant of arrest or a body attachment may not be issued based upon a failure to appear after being subpoenaed pursuant to this section.

A party requesting a continuance based upon the failure of a witness to appear in court at the time and place required for his or her appearance or testimony pursuant to a subpoena, shall prove to the court that the party has complied with the provisions of this section. Such a continuance shall only be granted for a period of time which would allow personal service of the subpoena and in no event longer than that allowed by law, including the requirements of Sections 861 and 1382.